cover from the USA

On 27th May I got a cover from the USA.

One stamp about the Surrender of Saratoga (issued 07-10-1977) and two from a set of ten about Botanical Art (issued 29-01-2016) were used on the cover. The postmark is about ZIP Code Day in Avoca, Iowa, the day when postcode and date match. Is this a thing in your country? I have already seen similar postmarks from various towns in the USA, but here in Germany there were maybe one or two in the last years. 

Inside was this card about the White Sands National Park in New Mexico, the largest gypsum dunefield in the World. Originally created as National Monument in 1933, it is a National Park since 2019.

Thank You very much Sam-Quito!